As a representative permanent magnet with high performance, the R-T-B based permanent magnet has an increasing production each year for its high magnetic properties, and it is used in; various applications such as various motors, various actuators, MRI apparatus and the like. In the R-T-B based permanent magnet, R is at least one selected from rare earth elements, T is Fe or a combination of Fe and Co, and B is boron.
In recent years, with the popularity of the hybrid electric vehicle (HEV), the demand for the R-T-B based permanent magnet used in the motor/generator of HEV is increased. In the above applications, the magnet is exposed in a relative high temperature and has a problem of demagnetization at high temperature caused by heat, thus, a permanent magnet having a high maximum energy product and a high heat resistance is preferred. In order to maintain the high magnetic properties even under a high temperature, at method of sufficiently increasing the coercivity of the R-T-B based sintered magnet under room temperature is known to be effective.
For example, in the Patent Document 1, the coercivity under room temperature is increased by controlling the grains and the grain boundaries, and a maximum high coercivity of about 30 kOe is obtained. However, a permanent magnet with even higher properties is required.
In addition, permanent magnets with a high coercivity other than the R-T-B based permanent magnet are proposed. In Patent Document 2, in a permanent magnet with Sm5Fe17 intermetallic compound as the main phase, a quite high coercivity of 37 kOe under room temperature is obtained. Further, 1 at % of C or B or the like is added and enters into the grain boundary part, causing the micronization of the main phase grains and thus a permanent magnet with good magnetic properties is obtained.
However, as described in Non-Patent Document 1, the Sm5Fe17 intermetallic compound has Curie temperature of about 270° C. which is lower than that of the representative R-T-B based permanent magnet of Nd2Fe14B. Thus, it is easy to demagnetize at high temperature and does not suit for an application which demands for high properties under high temperature. Even in the samples added with 1 at % of C or B or the like, the improvement for the demagnetization at high temperature still cannot be said to be sufficient. The improvement for the demagnetization at high temperature of Sm5Fe17 intermetallic compound is useful because the high coercivity under room temperature can be effective utilized till high temperature.